In the small backroom of Capitol Hemp, a head shop in Adams Morgan, a worker dutifully arranges an array of ceramic pipes displayed in a well-lit glass case. Another clerk helps a couple of customers as they peruse a selection of bongs and vaporizers.

Stored behind the counter is another amply stocked product whose popularity is booming: "spice," the generic name for a legal "synthetic marijuana." Capitol Hemp owner Adam Eidinger said that in the 18 months since he began stocking spice, demand has doubled each month, and its sales now represent a third of his revenue. On some Fridays, he said, his two District stores can bring in $10,000 from the sale of spice alone.

In the District and most states across the country, it is legal to buy and sell spice, whose crushed green leaves are sprayed with various man-made chemicals. When smoked, the treated leaves can produce a marijuana-like high.

But alarmed by its growing use and questions about its safety, lawmakers in a number of states have begun taking action.

Last week, Missouri Gov. Jay Nixon (D) became the latest to sign a state ban. In March, Kansas was the first state to outlaw the product, followed by Kentucky, Alabama, Tennessee and Georgia. Lawmakers in other states, including Iowa, Michigan, Illinois and Louisiana, are working on bans. Similar legislation has not come up in Virginia, Maryland or the District.

Gil Kerlikowske, President Obama's drug czar, said in an interview that the substance is "on our radar" but added that he thought state legislatures are dealing well with the issue.

But others decry what they see as a knee-jerk reaction from lawmakers, making the synthetic marijuana product the latest substance at the center of an ongoing debate about the merits of prohibition.

"We have never had any complaints or concerns from customers," Eidinger said. He added that he began stocking spice products after several requests from customers. "We always ask the manufacturers if there is anything illegal in the products. We only use the products we trust, and if it is made illegal in D.C., we will stop selling it."

At his shop, customers show ID to prove that they are 18 or older, then enter a room where they can study a sheet of paper listing the available brands of spice. For $55, they can buy three grams of K2 Summit, packaged in shiny foil. Those wanting a fruity option can go for Pep Pourri at $22.50 a gram.

Scott Rupp, a Missouri state senator, said he backed the ban for good reason. "We were getting reports from local law enforcement that this was exploding among the youth population," the Republican said. "We were getting reports of kids hurting themselves and showing up in the emergency room as they were sick from it."

The fact that spice cannot be detected by drug screening has also made it popular with other groups, including parolees, according to drug experts. Eidinger said many of his customers are in the armed forces. "They sometimes buy a $400 batch before going on tour," he said.

A lack of data and controlled testing make it difficult to determine the drug's safety. And there are no official estimates of its growing use. But there has been a significant bump in calls to poison centers concerning spice. Nationwide, the American Association of Poison Control Centers logged 567 cases across 41 states in which people had suffered a bad reaction to spice during the first half of 2010. Just 13 cases were reported in 2009.